1. Field
Embodiments of the invention are related to inspecting the interface or contact between lithographic resist film (e.g., dry film resist (DFR)) and conductive traces formed on the top surface of an insulating substrate such as for a chip package, for film footing defects and conductive trace undercut (CUT) defects.
2. Description of Related Art
One of the key issues resulting in high yield loss in substrate package technology development (SPTD) or bump-less build-up layer (BBUL) packaging is missing or lifted conductor (e.g., Copper—Cu) traces formed on the top surface of an insulating substrate (e.g., microprocessor packaging substrate, or a substrate having a surface of dielectric, insulator, or ajinomoto build-up film (ABF)). Yield loss as high as 70% due to this issue has been observed. Key contributors for missing or lifted conductive traces include “hidden defects” such as dry film resist (DFR) footing defects of film patterned to form the traces; and conductive trace undercut (CTU) defects of traces formed on the substrate. Large DFR and CTU defects can result in smaller contact area between conductive traces and insulating substrate, and thus cause conductive trace to lift off in downstream process. Such liftoff can result in damaged, destroyed or missing lengths of the conductive traces, often causing undesired open circuits in the trace circuitry.
Therefore, inspecting DFR footing and CTU can be a key step for process control and prevention of yield loss and reliability issues during SPTD process development. Developing non-destructive non-contact measurement solution for such defects is inherently challenging due to the hidden nature and small dimension of the defects. Current inline monitoring tools (e.g., tools that can be used while or after the traces are formed and prior to subsequent processing of the package) such as X-ray and acoustic microscopy do not have the capability to image and screen such defects due to low contrast or resolution of the interface or contact between conductive traces and substrate. So far the only method available to assess DFR and CTU defects is non in-line, physical failure analysis (FA), cross-section which is destructive to the substrate and has low-throughput due to the time required to cut the substrate. Thus, this method is not amenable to inline process monitoring or inspection of the interface, such as inspection during manufacture or processing to create packages during SPTD or BBUL. What is needed is a high-throughput and low cost metrology tool for non-destructive, non-contact in-line monitoring of both DFR and CTU defects.